This invention relates to communication between devices in a system, and more particularly to methods and apparatus for automatically adjusting transmission and/or reception of signals to compensate for deficiencies in the communication link being used.
Circuitry via which it is desired to send a signal may have various deficiencies that adversely affect transmission of the signal. For example, the communication link may somewhat attenuate (weaken) the signal and/or the communication link may introduce a phase shift into the signal. The magnitudes of these various deficiencies (e.g., attenuation and/or phase shift) may depend on the frequency(ies) of the signal being transmitted. These deficiencies can make it more difficult or even impossible to reliably receive a signal that has been transmitted, especially at certain frequencies or in certain frequency ranges.
Pre-emphasis and equalization are known techniques for attempting to compensate for deficiencies in a communication link. Pre-emphasis is used at the transmitter and involves giving the signal transmitted some extra energy for a certain amount of time after each change in level of the signal. Equalization is used at the receiver and involves initially responding more strongly to each change in level of the received signal. Each communication link is, however, potentially different. Also, some components (e.g., programmable logic devices or PLDs) are manufactured to be general-purpose devices to at least some degree, so that it may not be known by the manufacturer what use will be made of those devices (e.g., what communication links and frequencies they will be used with). It can accordingly be difficult or impossible to build into communication devices the appropriate amount of pre-emphasis and/or equalization.